1. Field of the Invention
In general, the present invention relates to a recording-medium library apparatus comprising a rotary cell drum having a plurality of cells each for accommodating a recording-medium cartridge. In particular, the present invention relates to a relative-position measuring system of a recording-medium library apparatus.
2. Description of the Related Art
A magnetic-tape drive is available as one of external storage devices of a computer. Most of the magnetic-tape drives adopt an open-reel technique using a tape with a width of 0.5 inches. However, the operation of such a magnetic-tape drive is cumbersome. Therefore, an autoloading apparatus, that is capable of automatically mounting a tape once the reel is set on a pan, is becoming popular. On the other hand, also widely used is a magnetic-tape recording-medium cartridge which utilizes a tape with the same width as the open reel and automatically pulls the tape from a container to be loaded into a magnetic-tape drive so as to reduce the amount of a work load borne by the operator.
As many magnetic-tape recording-medium cartridges as required are entered in a magnetic-tape library apparatus and selectively loaded into a magnetic-tape drive unit for allowing data to be written into or read out from the loaded magnetic-tape recording-medium cartridge. The magnetic-tape library apparatus comprises a cell unit having a plurality of cells each for accommodating a magnetic-tape recording-medium cartridge, a magnetic-tape drive unit for writing or reading out data into or from a loaded magnetic-tape recording-medium cartridge and an accessor for conveying a magnetic-tape recording-medium cartridge between the cell unit and the magnetic-tape drive unit.
The magnetic-tape library apparatus further includes a recording-medium cartridge access station (CAS) provided therein for entering and ejecting a magnetic-tape recording-medium cartridge. The CAS comprises a recording-medium cartridge entering unit and recording-medium cartridge ejecting unit. In recent years, such a magnetic-tape library apparatus is widely used as a large-capacity external storage device of a computer. It is necessary for such a magnetic-tape library apparatus to operate with a high degree of reliability even if the magnetic-tape library apparatus is used as an unattended system.
A rotary cell drum which is rotated by a driving means is known as one of cell units. Before a magnetic-tape recording-medium cartridge is entered by the accessor into a target cell of the cell drum, the cell drum is rotated so that the target cell is moved to a position that can be accessed by the accessor. A large-size magnetic-tape library apparatus comprises a plurality of independent frames which are connected to each other. Each frame comprises, among other components, a cell drum and a magnetic-tape drive unit. The number of geometrical errors which result among components composing the mechanism when such a large-size magnetic-tape library apparatus is assembled is large, making it impossible to accurately position a cell of a cell drum with respect to the accessor by merely relying on mechanical design values. In order to solve this problem, relative-position measurement is carried out as a means for correcting such a positional discrepancy.
In the conventional relative-position measuring system, a plurality of edges of a cell drum having a heptagonal cross section are detected by means of a flag sensor installed ll hand mechanism of an accessor in order to measure the positions of the cells of the cell drum relative to the accessor. In such a conventional relative-position measuring system, since a relative position is measured by detecting the edge of a cell drum by using a reflective-type sensor as a flag sensor, the accuracy of the measurement is poor and, in some cases, the relative-position measurement ends in a failure.
In the case of a small-size magnetic-tape library apparatus, the density of cells in the cell drum is not so high so that there is a sufficient gap for entering the hand of the accessor in each cell. As a result, the accessor can be positioned correctly with respect to each cell even if the relative-position measurement itself is considered to be rough. In the case of a large-size magnetic-tape library apparatus, on the other hand, a plurality of cells need to be laid out in the cell drum at a high density, making it impossible to provide a large space for entering the hand of the accessor into each cell.
As a result, the magnitude of an error between a position of a cell relative to the accessor as recognized by the accessor and the actually measured relative position increases, giving rise to a problem that the recording-medium cartridge can not be inserted in or taken out from a cell. This problem is a specially serious problem for a large-size magnetic-tape library apparatus which comprises a plurality of independent frames connected to each other because a connection error among the frames also exists. It is therefore necessary to measure a position of each cell of a cell drum relative to an accessor with a high degree of accuracy.